The invention relates to a chuck with triaxial construction comprising a receiving surface for a test substrate wherein there is arranged below the receiving surface an electrically conductive first surface element (force), an electrically conductive second surface element (guard) electrically insulated therefrom, and an electrically conductive third surface element (shield) electrically insulated therefrom, and, between the first and the second surface element, a first insulation element and, between the second and the third surface element, a second insulation element.
For testing semiconductor chips or similar substrates, needles are used to make contact with contact islands on the substrate surface and said contact islands are electrically connected via said needles to test devices used to measure the properties of the substrates. So-called probers having a clamping table for fixedly holding the substrates, a so-called chuck, are used for this purpose. Such a chuck is usually also provided with vacuum clamping means for fixedly holding the substrates on the substrate surface.
During testing it may also become necessary to carry out very low current measurements. Since, during these very low current measurements, tiny leakage currents lead to a corruption of the measurement result, the components that are at least indirectly connected with the measuring operation, such as lines, the chuck or the like, are provided with a triaxial construction. This means that the components which are at measurement potential (force) not only have a shield which is connected to earth or is embodied in floating fashion, rather an additional shield, a so-called guard, to which is applied a potential which at least approximately corresponds to the force potential that is decoupled from the latter, is also arranged between force and shield.
Accordingly, for very low current measurements, the chuck, too, is constructed in triaxial fashion. It has a top side, on which the substrate bears and to which force potential is applied. The guard extends below the top side, said guard being isolated from the top side by an insulating layer. The shield is then situated below the guard—likewise in a manner electrically insulated from the guard.
This construction can be realized in the form of differently conductive layering of a ceramic chuck. AlN, for example, is suitable for this as ceramic material. It is found here that the contact made with these layers externally is poor. This also applies to layers or interlayers applied by sputtering or paper deposition.
Moreover, in this case the required insulation resistance often cannot be achieved. Other ceramic materials having a sufficient insulation resistance, such as BN (boron nitride), for example, in turn cannot be provided with conductive layers or can only be poorly provided with conductive layers.
It is also possible to realize a triaxial construction by means of metal plates insulated from one another. In the case of chucks constructed in this way, the thermal mass is unnecessarily high. Moreover, difficulties occur in producing a basic flatness and maintaining this flatness under thermal loading.
In all solutions, besides the effectiveness of the triaxial construction, it is also necessary to avoid an adverse influence of the triboelectric effect. In this case, friction between layers having different dielectric constants, for example metal and ceramic, as a result of a triboelectricity generates a charge that can ultimately corrupt the measurement result.